IB Psychology HLTopic 1 — Health ProblemsPaper 1 & 2Core skill~8 min read
How Common Is Obesity?
Before anyone can explain a health problem, they have to count it. Prevalence is that counting exercise, and it sounds like the least interesting part of the topic. It is actually where most of the argument happens, because how you measure obesity decides what the number turns out to be.
📘 What you need to know
Prevalence is how common something is in a defined population over a set period of time.
You calculate it by dividing the number of cases by the total population. 4,500 cases in a population of 50,000 gives a prevalence of 9%.
Prevalence is a predictive tool: it tells you the likelihood that any given person in that population is affected.
Obesity is usually defined as a Body Mass Index (BMI) over 30.
UK figures for 2023/24 reported around 26.5% of adults as obese, and 64.5% as overweight or obese combined.
Kyle et al. (2016) compared 13,483 Scottish adults and found nurses had the highest rates among the groups studied.
The core criticism: BMI is a rough measure and prevalence data tells you what, never why.
Calculating prevalence
Prevalence rate
prevalence = number of cases ÷ total population
The number can be reported as a percentage, or as cases per 10,000 or per 100,000 people. Which one you choose depends on how common the condition is — percentages suit common problems, cases per 100,000 suit rare ones.
Prevalence always needs three things attached to it: a population, a time period, and a definition. “26.5% are obese” is meaningless without “UK adults, 2023/24, BMI over 30”. Write all three in the exam.
Research support: Kyle et al. (2016)
Aim: to investigate obesity prevalence among nurses in Scotland compared with other professions.
Participants: 13,483 adults aged 17–65, made up of 411 nurses, 320 other healthcare professionals, 685 care assistants and 12,067 people working outside healthcare.
Procedure: participants’ BMI was measured.
Results: nurses came out highest at 69.1%, compared with 68.9% for non-healthcare workers, 68.5% for care assistants and 51.3% for other healthcare professionals.
Conclusion: rates among Scottish nurses were significantly higher than in the other healthcare groups studied.
Look at the shape of the data rather than the headline. The interesting question is not why nurses are highest, but why one healthcare group sits so far below everyone else.
Two things worth knowing about these figures. First, they cover overweight and obesity together rather than obesity on its own, which is why they look so high next to the 26.5% UK obesity figure. Second, the gap between nurses and non-healthcare workers is 0.2 percentage points — noticing that is exactly the kind of critical reading examiners reward.
Evaluation of prevalence research
Strengths
Very useful for planning. Health services need to know the size of a problem before they can budget for it, and obesity is linked to serious health risks.
Large samples. Over 13,000 participants produces robust quantitative data that is reliable and reasonably generalisable within Scotland.
Measured, not self-reported. BMI was actually taken rather than asked about, which removes a major source of bias.
Limitations
Knowing is not fixing. Prevalence tells governments the scale of the problem but not what intervention would work.
No depth. The data cannot explain why rates differ. Qualitative research is needed to get at lifestyle, shift patterns, income and culture.
BMI is a blunt instrument. It uses only height and weight, so it ignores muscle mass, bone density and differences between population groups. It cannot identify body fat with full accuracy.
Group sizes were very unequal. 411 nurses against 12,067 non-healthcare workers means the smaller figures are far less stable.
EXAM QUESTION
Discuss the prevalence of one health problem. [22]
Step 1: define prevalence and show the calculationCases divided by population, over a stated time period, in a stated group.Step 2: define the health problem itselfObesity = BMI over 30. Give the UK figures as an example.Step 3: use Kyle in fullSample, measured BMI, the four group percentages, the conclusion.Step 4: attack the measure and the methodBMI validity, unequal group sizes, no explanation of causes, one country only.Best closing point: prevalence describes, it never explains
Link to concepts
Measurement
BMI is convenient, cheap and standardised, which is why it is used everywhere. It is also not a direct measure of body fat, and it is not a “one size fits all” tool. Every prevalence figure on this page inherits that weakness, and a good essay says so before quoting a single number.
Responsibility
Obesity research is socially sensitive. Published figures can be used by the media to attribute blame to individuals without acknowledging the factors behind them — income, stress, food availability, education, working patterns. Researchers carry a responsibility to report findings in that context rather than as a lifestyle choice.
💡 Exam tip
Always show the calculation once. It takes one line and proves you understand the term rather than just recognising it.
Quote the population and the year with every statistic.
The BMI criticism is the single most useful evaluation point in this whole sub-topic, and it works for every obesity study.
Compare group sizes when a study has several groups. Unequal samples are a real methodological weakness.
Keep your language neutral and non-judgemental. It reads as more professional and it is also more accurate.
Prevalence questions almost always want the “descriptive not explanatory” distinction. Learn that phrase.
⚠ Common mix-up
Prevalence versus incidence. Prevalence counts all existing cases; incidence counts only new ones in a period.
Treating BMI as a measure of body fat. It is a ratio of weight to height, nothing more.
Mixing “obese” with “overweight or obese”. The two figures are very different and students constantly quote the wrong one.
Saying prevalence data explains causes. It cannot, by design.
Generalising Scottish data to the world. One country, one health system, one set of working conditions.
Forgetting that BMI was measured, not self-reported. That is a strength and it is worth a mark.
Up next: How Common Is Smoking? — same tools, a very different pattern, and a global trend that is actually heading in the right direction.
Want this explained one-to-one?
Book a free session with an experienced IB Psychology tutor and get your trickiest topics made simple.
